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Air well (condenser)
in Trans-en-Provence.]] 117,000.|A radiative condenser in northwest India. ]] An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture from air. Designs for air wells are many and varied, but the simplest designs are completely passive, require no external energy source and have few, if any, moving parts. A related, but quite distinct, technique of obtaining atmospheric moisture is the fog fence. An air well should not be confused with a dew pond. A dew pond is an artificial pond intended for watering livestock. The name dew pond (sometimes cloud pond or mist pond) derives from the widely held belief that the pond was filled by moisture from the air.Oxford English Dictionary: "dew-pond" In fact, dew ponds are primarily filled by rainwater.Pugsley, 1939 All air well designs utilise a substrate with a temperature sufficiently low so that dew forms. This condensation releases latent heat which must be dissipated. Cooling of the substrate is typically achieved by radiating heat to the sky, during periods of cool breezes (particularly at night), or by conduction to some other heat sink. Three principal designs are used for air wells: high mass, radiative and active. High-mass air wells were used in the early 20th century, but the approach failed. From the late 20th century, onwards, low-mass, radiative collectors proved to be much more successful. Thirdly, active collectors collect water in the same way as a dehumidifier, although the designs work well, they require an energy source making them uneconomical except in special circumstances. New, innovative designs seek to minimise the energy requirements of active condensers or make use of renewable energy resources. __TOC__ Moisture An air well requires moisture from the air; and everywhere on Earth, even in the hottest climates, the surrounding atmosphere contains at least some water. According Beysens and Milimouk: "The atmosphere contains of fresh water, composed of 98% water vapour and 2% condensed water (clouds): a figure comparable to the renewable liquid water resources of inhabited lands ." The quantity of water vapour contained within the air is commonly reported as a relative humidity; and this depends on temperature – warmer air can contain more water vapour than cooler air. When air is cooled to the dew point it becomes saturated and moisture will condense on a suitable surface.Naval Meteorology and Oceanography Command (2007). Atmospheric Moisture. United States Navy. Retrieved on 2008-12-27. For instance, the dew temperature of air at and 80% relative humidity is . The dew temperature falls to if the relative humidity is only 25%. An effective air well must provide a substrate at a temperature that is sufficiently low to allow dew to form upon its surface. Dew is a form of precipitation that occurs naturally when atmospheric water vapour condenses onto a substrate. It is distinct from fog, in that fog is made of droplets of water that condense around particles in the air. Condensation of the dew releases latent heat which must be dissipated in order for water collection to continue.Nikolayev et al, 1996. p.23-26 The substrate must be cooled; this is typically achieved by radiating heat to the sky, by periods of cool breezes (particularly at night) or by conduction to some other heat sink. A stone mulch can significantly increase crop yields in arid areas. This is most notably the case in the Canary Islands: on the island of Lanzarote there is about of rain each year and there are no permanent rivers. Despite this, substantial crops can be grown by using a mulch of volcanic stones, a trick discovered after volcanic eruptions in 1730. Some credit the stone mulch with promoting dew; although the idea has inspired some thinkers, it seems unlikely that the effect is significant. Rather, plants are able to absorb dew directly from their leaves, and the main benefit of a stone mulch is to reduce water loss from the soil and to eliminate competition from weeds. Heat sinks There are three principal approaches to the design of the heat sinks that collect the moisture in the air wells: high mass, radiative and active. Early in the twentieth century, there was interest in high-mass air wells, but despite much experimentation including the construction of massive structures, this approach proved to be a failure.Alton, 2003. p. 1014. From the late twentieth century onwards, there has been much investigation of low-mass, radiative collectors; these have proved to be much more successful. Also, since the invention of refrigeration systems there has been interest in active collectors that collect water in the same way as a dehumidifier; although these designs work well, they require an energy source which makes them uneconomical except in special circumstances. Newer, innovative designs seek to minimise the energy requirements of active condensers or make use of renewable energy resources. High-mass collectors A number of inventors experimented with high-mass collectors. Notable investigators were the Russian engineer Friedrich Zibold (sometimes given as Friedrich Siebold), the French bioclimatologist Leon Chaptal, the German-Australian researcher Wolf Klaphake and the Belgian inventor Achille Knapen. s in diameter at the base and in diameter at the top. (b) is a concrete bowl; a pipe (not shown) leads away from the base of the bowl to a collecting point. © is ground level and (d) is the natural limestone base.Based on diagram by Nikolayev et all, 1996]] ;Zibold’s collector: In 1900, near the site of the ancient Byzantine city of Theodosia, thirteen large piles of stones were discovered by Zibold who was a forester and engineer in charge of this area.Nikolayev et al, 1996. p. 20-23 Each stone pile covered just over and was about tall. The finds were associated with the remains of diameter terracotta pipes that apparently led to wells and fountains in the city. Zibold concluded that the stacks of stone were condensers that supplied Theodosia with water; and calculated that each air well produced more than each day. To verify his hypothesis Zibold constructed a stone-pile condenser at an altitude of on Mt. Tepe-Oba near the ancient site of Theodosia. Zibold’s condenser was surrounded by a wall high, wide, around a bowl-shaped collection area with drainage. He used sea stones in diameter piled high in a truncated cone that was in diameter across the top. The shape of the stone pile allowed a good air flow with only minimal thermal contact between the stones.Nelson, 2003 Zibold's condenser began to operate in 1912 with a maximum daily production that was later estimated to have been – Zibold made no public record of his results at the time. The base developed leaks that forced the experiment to end in 1915 and the site was partially dismantled before being abandoned. (The site was rediscovered in 1993 and cleaned up.) Zibold's condenser was of approximately the same size as the ancient stone piles that had been found and although the yield was very much less than the yield Zibold had calculated for the original structures, the experiment was an inspiration for later developers. ;Chaptal’s collector: Inspired by Zibold's work, Chaptal built a small air well near Montpellier in 1929. Chaptal's condenser was a pyramidal concrete structure square and high, it was filled with of limestone pieces being about in diameter. Small vent holes ringed the top and bottom of the pyramid. These holes could be closed or opened as required to control the flow of air. The structure was allowed to cool during the night and then warm moist air was let in during the day. Dew formed on the limestone pieces and collected in a reservoir below ground level. The amount of water obtained varied from to per day depending on the atmospheric conditions.Hills, 1966. p. 232. Chaptal did not consider his experiment a success. When he retired in 1946, he put the condenser out of order, possibly because he did not want to leave an improper installation to mislead those who might later continue studies on air wells.Beysens et al 2006 ;Klaphake’s collectors: Klaphake was a successful chemist working in Berlin during the 1920s and 30s. During that time, he tested several forms of air wells in Yugoslavia and on Vis Island in the Adriatic Sea. Klaphake's work was inspired by the works of Maimonides, a known Jewish scholar who wrote in Arabic about 1,000 years ago and who mentioned the use of water condensers in Palestine. Klaphake experimented with a very simple design: an area of mountain slope was cleared and smoothed with a watertight surface with a simple canopy supported by pillars or ridges. The sides of the structure were closed, but the top and bottom edges were left open. At nights, the mountain slope would cool and in the day moisture would collect on and run down the smoothed surface. Although the system apparently worked, it was expensive and Klaphake finally adopted a more compact design based on a masonry structure. This design was a sugarloaf-shaped building, about high, with walls at least thick, with holes on the top and at the bottom. The outer wall is made of concrete to give a high thermal capacity and the inner surface was made of a porous material such as sandstone.Sharan, 2006. p. 72. According to Klaphake: Traces of Klaphake's condensers have been tentatively identified. In 1935, Wolf Klaphake and his wife Maria emigrated to Australia. The Klaphakes' decision to emigrate was probably primarily the result of Maria's encounters with Nazi authorities; their decision to settle in Australia (rather than, say, in Britain) was influenced by Wolf's desire to develop a dew condenser. As a dry continent, Australia was likely to need alternative sources of fresh water and the Premier of South Australia, whom he had met in London, had expressed an interest. Klaphake made a specific proposal for a condenser at the small town of Cook where there was no supply of potable water. At Cook, the railway company had previously installed a large coal-powered active condenser, but it was prohibitively expensive to run, and it was cheaper to simply transport water. However, the Australian government turned down Klaphake's proposal and he lost interest in the project. ;Knapen’s ariel well: Knapen, who had previously worked on systems for removing damp from buildings, Prevention Of Damp In Buildings. The Manchester Guardian, 27 February 1930 p. 6 column F. was in turn inspired by Chaptal's work and he set about building an ambitiously large puits aerien (aerial well) on a high hill at Trans-en-Provence in France. Beginning in 1930, Knapen's dew tower took 18 months to build; it still stands today, albeit in dilapidated condition. At the time of its construction, the condenser excited some public interest. The tower is high and has massive masonry walls about thick with a number of apertures to let in air. Inside there is a massive column made of concrete. At night, the whole structure is allowed to cool, and during the day warm moist air enters the structure via the high apertures, cools, descends, and leaves the building by the lower apertures. Knapen’s intention was that water should condense on the cool inner column. In keeping with Chaptal’s finding that the condensing surface must be rough and the surface tension must be sufficiently low that the condensed water can drip, the central column's outer surface was studded with projecting plates of slate. The slates were placed nearly vertically to encourage dripping down to a collecting basin at the bottom of the structure. Unfortunately, the aerial well never achieved anything like its hoped-for performance and produced no more than a few litres of water each day. ;Conclusions None of the high-mass collectors performed well, and Knapen's aerial well was a particularly conspicuous failure. Although ancient air wells are frequently mentioned in sources, there is scant evidence for them and persistent belief in their existence has the character of a modern myth.Beysens et al 2006 Zibold's collector apparently performed reasonably well, but in fact his exact results are not at all clear and it is possible that the collector was intecepting fog which added significantly to the yield. Furthermore, it is now apparent that the mounds that Zibold identified as dew condensers were ancient burial mounds (a part of the necropolis of antic Theodosia) and that the pipes were medieval in origin and not associated with the mounds. If Zibold's condenser worked at all this was probably due to fact that a few stones near the surface of the mound were able to lose heat at night while being thermally isolated from the ground; however, it could never have produced the yield that Zibold envisaged.Beysens et al 2006Nikolayev et al, 1996 The problem with the high-mass collectors was that they could not get rid of sufficient heat during the night – despite design features intended to ensure that this would happen. While some thinkers have occasionally been persuaded that Zibold might have been on the right track after all, the reasoning of an article in Journal of Arid Environments makes it clear that high-mass condenser designs of this type are doomed to failure: In retrospect, it may be seen as unfortunate that Zibold's pioneering work inspired so much fruitless effort. However, Zibold's legacy would inspire one more group of people, a party of academics who would go on to form the International Organization For Dew Utilization and, taking a quite different approach, they designed successful condensers. Radiative collectors By the end of the twentieth century, the details of how dew condenses was much better understood, the key insight being that low-mass collectors that rapidly lose heat by radiation performed best. A number of researchers worked on this method.Sharan, 2006. p. 22. In the 1960s, simple dew condensers made from sheets of polyethylene were used in Israel to irrigate plants, and in 1986 in New Mexico condensers made of a special foil produced sufficient water to supply young saplings. ; International Organization for Dew Utilization In 1992 a party of French academics attended a condensed matter conference in the Ukraine where physicist Daniel Beysens introduced them to the story of how ancient Theodosia was supplied with water from dew condensers. They were sufficiently intrigued that in 1993 they went to see for themselves. The supposed dew condensers turned out to be burial mounds, but they also found the remains of Zibold's condenser which they tidied up and examined closely. Fired with enthusiasm, the party returned to France and set up the International Organisation for Dew Utilization (OPUR) with the specific objective of making dew available as an alternative source of water. OPUR began a study of dew condensation under laboratory conditions, they developed a special hydrophilic film and experimented with trial installations, including a collector in Corsica. Vital insights included the idea that the mass of the condensing surface should be as low as possible so that it cannot easily retain heat, that it should be protected from unwanted thermal radiation by a layer of insulation, and that it should be hydrophilic so as to shed condensed moisture readily.Sharan, 2006. pp. 20-28. By the time they were ready for their first practical installation, they heard that one of their members, Girja Sharan had obtained a grant to construct a dew condenser in Kothara, India. In April 2001, he had incidentally noticed substantial condensation on the roof of a cottage at Toran Beach Resort in the arid coastal region of Kutch, where he was briefly staying. The following year, he investigated the phenomenon more closely and interviewed local people. Financed by the Gujarat Energy Development Agency and the World Bank, Sharan and his team went on to develop passive, radiative condensers for use in the arid coastal region of Kutch.Sharan, 2006. Acknowledgement Active commercialisation began in 2006. Sharan tested a wide range of materials and got good results from galvanised iron and aluminium sheets, but found that sheets of a special plastic developed by the OPUR just thick generally worked even better than the metal sheets and were less expensive.Sharan, 2006. p. 27. The plastic film, known as OPUR foil, is hydrophilic and is made from polyethylene mixed with titanium oxide and barium sulphate. A typical radiative collector presents a condensing surface at an angle of 30° from the horizontal. The condensing surface is backed by a thick layer of insulating material such as polystyrene foam and supported above ground level. Such condensers may be conveniently installed on the ridge roofs of low buildings or supported by a simple frame.Sharan, 2006. pp. 20-39. Although other heights do not typically work quite so well, it may be less expensive or more convenient to mount a collector near to ground level on a two-storey building.Sharan, 2006. pp. 40-59. A radiative collector should be thermally isolated from any mass, including the ground. The condensing surface should be left open to radiate heat into space and should be well away from any source of heat or anything that can reflect heat radiation into it. Ideally, the condensing surface should be well wetted to reduce the nucleation barrier. The radiative condenser illustrated near the start of this article is built near the ground. In the area of north-west India where it is installed dew occurs for 8 months a year, and the installation collects about of dew water over the season with nearly 100 dew-nights. In a year it provides a total of about of potable water for the school which owns and operates the site. Although flat designs have the benefit of simplicity, other designs such as inverted pyramids and cones can be significantly more effective. This is probably because the designs shield the condensing surfaces from unwanted heat radiated by the lower atmosphere and being symmetrical they are not sensitive to wind direction.Clus, et al. New materials may make even better collectors.Sharan, 2006. p. 20. One such material is inspired by the Namib Desert beetle, which survives only on the moisture it extracts from the atmosphere. It has been found that its back is coated with microscopic projections: the peaks are hydrophilic and the troughs are hydrophobic. Researchers at the Massachusetts Institute of Technology have emulated this capability by creating a textured surface that combines alternating hydrophobic and hydrophilic materials. Active collectors Active atmospheric water collectors have been in use since the commercialisation of mechanical refrigeration. Essentially, all that is required is to cool a heat exchanger below the dew point and water will be produced. Such water production may take place as a by-product, possibly unwanted, of dehumidification. Because mechanical refrigeration is energy intensive, such active systems are typically restricted to places where there is no supply of water that can be desalinated or purified at a lower cost and that are sufficiently far from a supply of fresh water to make transport uneconomical. Such circumstances are uncommon, and even then large installations such as that tried in the 1930s at Cook in South Australia failed because of the cost of running the installation – it was cheaper to transport water over large distances. In the case of small installations, convenience may outweigh cost. There is a wide range of small machines designed to be used in offices that produce a few litres of drinking water from the atmosphere. However, there are circumstances where there really is no source of water other than the atmosphere. For example, in the 1930s, American designers added condenser systems to airships – in this case the air was that emitted by the exhaust of the engines and so it contained additional water as a product of combustion. The moisture was collected and used as additional ballast to compensate for the loss of weight as fuel was consumed. By collecting ballast in this way, the airship's buoyancy could be kept relatively constant without having to release helium gas which was both expensive and in limited supply.Allen, 1931. p. 37. More recently, on the International Space Station, the Zvezda module includes a humidity control system, the water it collects is usually used to supply the Elektron system that electrolyses water into hydrogen and oxygen, but it can be used for drinking in an emergency. There are a number of designs that minimise the energy requirements of active condensers: * One method is to use the ground as a heat sink by drawing air through underground pipes. This is often done to provide a source of cool air for a building by means of a ground-coupled heat exchanger (also known as Earth tubes) wherein condensation is typically regarded as a significant problem. A major problem with such designs is that the underground tubes are subject to contamination and difficult to keep clean. Designs of this type require air to be drawn through the pipes by a fan, but the power required may be provided (or supplemented) by a wind turbine. * Cold seawater is used in the Seawater Greenhouse to both cool and humidify the interior of greenhouse-like structure. The cooling can be so effective that not only do the plants inside benefit from reduced transpiration, but dew collects on the outside of the structure and can easily be collected by gutters. * Another type of atmospheric water collector makes use of desiccants which adsorb atmospheric water at ambient temperature, this makes it possible to extract moisture even when the relative humidity is as low as 14%. Systems of this sort have proved to be very useful as emergency supplies of safe drinking water. For regeneration, the desiccant needs to be heated, but in some designs this energy is supplied by the sun: air is ventilated at night over a bed of desiccants that adsorb the water vapour. During the day, the premises are closed, the greenhouse effect increases the temperature and, as in solar desalination pools, the water vapour is partially desorbed, condenses on a cold part and is collected. See also * Rainwater harvesting References Notes Sources * * * * * * * * * External links * OPUR (International Organisation for Dew Utilization) * * * * – KBTV: Aqua Sciences Emergency Water Station, that produces pure water out of the humidity-saturated air in the wake of a hurricane. Category:Precipitation Category:Water supply Category:Hydrology Category:Atmospheric sciences Category:Appropriate technology